1. Field of the Invention
The invention relates to a hybrid vehicle in which lean operation of an engine can be achieved at an air-fuel ratio higher than a stoichiometric air-fuel ratio. More specifically, the invention relates to a hybrid vehicle in which lean limit operation of an engine can be achieved while consumption of electric power of a battery is effectively suppressed.
2. Description of the Related Art
It is known that lean operation of an engine at an air-fuel ratio higher than a stoichiometric air-fuel ratio improves fuel efficiency. The fuel efficiency is gradually increased in an initial stage as the air-fuel ratio is increased from the stoichiometric air-fuel ratio. However, if the air-fuel ratio is extremely increased, a change amount of engine torque is increased, which makes rotation in the engine unstable, and deteriorates the fuel efficiency after all. Until the air-fuel ratio is increased to reach a limit, the fuel efficiency can be increased while a change amount of the engine torque is suppressed to be equal to or smaller than a permissible limit value. This limit is referred to as “a lean limit”. Various technologies relating to operation of an engine in the vicinity of a lean limit are proposed.
Japanese Patent No. 3289361 discloses a technology in which when an engine for electric power generation is operated in an electric vehicle, a change amount of engine torque is detected based on an output of an electric generator; and an air-fuel ratio of an engine is increased to a lean limit while determining whether the change amount of the engine torque is equal to or smaller than a value corresponding to a predetermined lean limit of the air-fuel ratio of the engine. Also, Japanese Patent No. 3307015 discloses a technology in which as operation control of a lean burn engine for a vehicle, a combustion pressure of an engine is detected; and when a change amount of the combustion pressure exceeds a predetermined value corresponding to a predetermined lean limit, a change in engine torque is suppressed using an electric motor. Also, Japanese Utility Model Application Publication No. 7-41880 discloses a technology in which in an engine using gas as fuel, an engine rotational speed is caused to come close to a target rotational speed; when the engine rotational speed matches the target engine rotational speed, a cross sectional area of a gas passage is adjusted so that the air-fuel ratio of air-fuel mixture becomes lean; and a decrease in the engine rotational speed caused by the operation of the engine at a lean air-fuel ratio is compensated for by adjusting a throttle valve opening amount. Also, Japanese Patent No. 3286517 discloses a technology in which in a vehicle including a lean burn engine and a continuously variable transmission, the continuously variable transmission is controlled according to an operating state of the vehicle so that an engine rotational speed becomes equal to a target engine rotational speed; and the target engine rotational speed is set to a value corresponding to a lean limit.
In a drive system of a hybrid vehicle that is employed by the applicant of the invention, an engine, a first motor-generator, a drive wheel are mutually connected via differential connection means; and a second motor-generator is connected to the drive wheel. In a case where this drive system is operated in the vicinity of a lean limit, a convenient method is as follows. A change amount of engine torque is detected using the first motor-generator, and using the drive wheel as a stable torque support member; and when the change amount of the engine torque exceeds a predetermined limit value, the change in the engine torque is compensated for using the second motor-generator, whereby a lean limit is controlled. This method is actually employed.
However, when the engine torque changes in the vicinity of the lean limit, the engine torque becomes intermittently insufficient. Therefore, the change in the engine torque is compensated for using the second motor-generator. Since the change in the engine torque is compensated for by operating the second motor-generator as a motor, electric power of a battery is consumed. Also, even in the case of lean operation of the engine, the engine cannot be operated at a lean air-fuel ratio in the initial stage when the engine is started. Therefore, the engine is always started in a rich state, that is, at an air fuel ratio equal to or lower than the stoichiometric air-fuel ratio. Then, after warming-up of the engine has proceeded to a certain extent, and the rotation in the engine has become stable, the air-fuel ratio is gradually increased so that the engine starts to be operated at a lean air-fuel ratio. However, since the engine torque is decreased with an increase in the air-fuel ratio, when the air-fuel ratio is increased toward a lean limit during the engine operation, the second motor-generator needs to be operated. Since the second motor-generator is operated, the electric power of the battery is further consumed.
When the vehicle is stably driven in a constant running state by the engine, the vehicle receives a constant output (power) from the engine. The output of the engine is a product of the torque and the rotational speed. Therefore, when the engine torque is decreased, if the engine rotational speed is increased with a decrease in the engine torque, the engine output can be maintained at a constant level. Accordingly, in the case where the vehicle includes a drive system in which the engine, the motor-generator, and the drive wheel are mutually connected via the differential connection means, when the engine torque is intermittently decreased in a lean operation region, or when the engine torque is gradually decreased with an increase in the air-fuel ratio toward a lean limit during the engine operation, the decrease in the engine torque should be able to be compensated for by increasing the engine rotational speed without changing the rotational speed of the drive wheel even if means such as the second motor-generator is not used.